Question: In geometry class, the girl to boy ratio is $9$ to $4$. If there are a total of $65$ students, how many girls are there?
Explanation: A ratio of $9$ girls to $4$ boys means that a set of $13$ students will have $9$ girls and $4$ boys. A class of $65$ students has $5$ sets of $13$ students. Because we know that there are $9$ girls in each set of $13$ students, the class must have $5$ groups of $9$ girls each. There is a total of $45$ girls in geometry class.